1. Field of the Invention
The present invention relates to an antenna for wireless communication, and particularly to a linear antenna such as a dipole antenna or monopole antenna.
2. Description of the Related Art
Various antennas are in use as antennas for ground wireless communication, one type being the dipole antenna, in which a cylindrical conductor is fed at its center. We refer now to FIG. 1, in which one example of the construction of a dipole antenna is shown. This dipole antenna is provided with two cylindrical conductors 60 in linear form each having a length of xc2xc wavelength, these cylindrical conductors 60 being configured so as to be fed by feeding point 61 placed in between them (Fujimoto, K. and James, J. R. Mobile Antenna Systems Handbook. Artech House, Norwood. 1994. pp. 462, 463). In this dipole antenna, a uniform radiation pattern is obtained by supplying a radio-frequency current from feeding point 61.
This type of dipole antenna is often used in, for example, wireless LANs (Local Area Networks), indoor communication systems, portable equipment, and portable telephones (such as a cellular phones), but since this antenna is a construction capable of uniform radiowave radiation to free space (nondirectivity), the presence of scattered objects in the direction of radiation (for example, a human body close to the antenna or obstructions to radiowaves in a wireless LAN environment) affects the radiation characteristics and bring about a reduction in gain and radiation efficiency. As a result, improvements in the radiation characteristics in wireless LANs and indoor communication systems have been sought by using antennas having directivity in a particular direction.
One example of antenna that is directional in any direction is a dipole antenna capable of varying the radiation pattern such as the antenna described in Japanese Patent Laid-open No. 307142/96. FIG. 2(a) shows the construction of this antenna, while FIG. 2(b) shows its radiation characteristics. As shown in FIG. 2(a), this dipole antenna is provided with: half-wave dipole antenna 70 and reflecting element 71 having an arc form and set at a prescribed distance from half-wave antenna 70. In this dipole antenna, electromagnetic waves emitted from half-wave dipole antenna 70 in the direction of point 0 are reflected in the direction of point 0xe2x80x2 by reflecting element 71, as shown in FIG. 2(b). As a result, the electromagnetic waves that are radiated toward point 0xe2x80x2 from half-wave dipole antenna 70 include direct electromagnetic waves that are emitted directly from half-wave dipole antenna 70 and reflected electromagnetic waves that are reflected by reflecting element 71. Although these direct electromagnetic waves and reflected electromagnetic waves combine, their phases diverge because their propagation distances are different, and this antenna therefore exhibits a dual directivity characteristic, as described in the publication.
As described in the foregoing explanation, dipole antennas of the prior art suffered from the problem that, when scattered objects are present in the direction of radiation, the scattered objects bring about a reduction in the radiation characteristic of the antenna.
Since the radiation characteristics of the dipole antenna described in Japanese Patent Laid-open No. 307142/96 can be varied by the reflecting element, the radiation pattern can be set to avoid obstructions and thus mitigate the influence of the above-described obstructions. However, such a dipole antenna is not capable of adequate adjustment of directivity in the horizontal plane and vertical plane, and moreover, its transmission frequency cannot adequately cope with a plurality of frequencies.
Based on the results of experimentation and analysis, the inventors discovered a further improvement in the radiation characteristics of a dipole antenna by addressing the following four points:
When a reflecting element (parasitic element) is provided at a position that is a fixed distance from a dipole antenna, impedance in the dipole antenna as seen from the feeding point varies under the influence of electromagnetic waves generated by the reflecting element. In order to solve this problem, matching is realized between the dipole antenna and a matching circuit that supplies a high-frequency current to the feeding point, thereby obtaining the original antenna characteristics.
When the length of reflecting element (parasitic element) diverges from the length of xcex/2 of the transmission frequency, the reflecting element does not contribute to the adjustment of directivity of the dipole antenna. To solve this problem, the reflecting element is set to the length of xcex/2 of the transmission frequency to allow sufficient adjustment of directivity.
Making the length of the reflecting element (parasitic element) variable allows handling a plurality of transmission frequencies. A dipole antenna that is capable of adjusting directivity and capable of handling a plurality of transmission frequencies has not been previously reported.
Adjustment of directivity is not only made possible in the horizontal plane of the dipole antenna, but in the vertical plane as well. Typically, in a case in which the antenna beam is directed below or above the horizontal plane, the dipole itself must be tilted. A construction in which the dipole is tilted and the beam cast up and down generally can only be realized by mechanical means, and such a structure not only raises the cost of the device, but is disadvantageous for realizing a more compact antenna.
It is an object of the present invention to provide a linear antenna based on the above-described views that is capable of adjusting directivity and that allows impedance matching.
It is another object of the present invention to provide a linear antenna that can handle a plurality of transmission frequencies.
It is yet another object of the present invention to provide a linear antenna that allows adjustment of both directivity in the horizontal plane and directivity in the vertical plane.
To achieve the above-described objects, the linear antenna of the present invention comprises: a linear radiation element; at least one linear parasitic element having a length of one half-wavelength of a prescribed transmission frequency and arranged parallel to the linear radiation element; and a U-shaped parasitic element arranged in proximity to one end of the linear radiation element.
In the case described above, a construction may be adopted in which a plurality of the linear parasitic elements is arranged in an arc so as to surround the linear radiation element.
In addition, the linear parasitic elements may each be constructed from a plurality of linear conductors that are connected via switch elements, wherein electrical connection can be effected between any adjacent linear conductors. In this case, the linear parasitic element may be constructed such that the length of linear conductors that are connected by all of the switch elements is one half-wavelength of a prescribed transmission frequency. Alternatively, the linear parasitic element may be constructed such that the length of linear conductors that are electrically connected by a portion of the switch elements is one half-wavelength of a prescribed transmission frequency.
In the above-described construction, a plurality of the linear parasitic elements may be arranged in an arc so as to partially surround the linear radiation element. Alternatively, a plurality of the linear parasitic elements may be arranged so as to completely surround the linear radiation element.
In any of the above-described constructions, the linear parasitic elements and the U-shaped parasitic element may each be printed on a plate composed of dielectric material.
Further, the U-shaped parasitic element may have two arms, and may be of a construction in which the arms are arranged parallel to the linear radiation element.
The present invention constructed according to the above description solves the above-described problems by exhibiting the following effects:
In the present invention, the length of linear parasitic elements that are provided around the circumference of a linear radiation element is one half-wavelength of the transmission frequency, and as a result, when a current is induced in the linear parasitic elements by the electromagnetic waves from the linear radiation element, a resonance current flows in the linear parasitic elements. Electromagnetic waves radiated from the linear parasitic elements due to this resonance current combine with electromagnetic waves radiated from the linear radiation element, thereby changing the directivity of the radiation.
In the present invention, moreover, a U-shaped parasitic element is arranged in proximity to one end of the linear radiation element, and this U-shaped parasitic element realizes impedance matching between the linear radiation element and the feed system. Accordingly, there is no divergence in impedance matching between the linear radiation element and feed system as in the prior art.
In cases of the present invention in which a plurality of linear parasitic elements are arranged in an arc around the circumference of the linear radiation element, a radiation pattern having stronger directivity can be realized because the electromagnetic waves radiated from each of the linear parasitic elements that are arranged in an arc combine with electromagnetic waves that are radiated from the linear radiation element.
In cases of the present invention in which the linear parasitic elements are composed of a plurality of linear conductors that are connected via switch elements, the length of the linear parasitic elements can be varied by ON/OFF control of the switch elements, whereby the length of the linear parasitic elements can be set in accordance with a plurality of transmission frequencies.
In cases of the present invention in which a plurality of linear parasitic elements, which are composed of a plurality of linear conductors that are connected via switch elements, are arranged in an arc around the circumference of the linear radiation element, the length of any of the linear parasitic elements can be set to a half-wavelength of the transmission frequency by ON/OFF control of the switch elements. As a result, adjustment of directivity in the horizontal plane can be realized for a particular portion of the bearings of the antenna.
In cases of the present invention in which a plurality of linear parasitic elements, which are composed of a plurality of linear conductors that are connected via switch elements, are arranged around the entire circumference of the linear radiation element, adjustment of directivity in the horizontal plane can be realized for all bearings of the antenna due to the same effect as described above.
In cases of the present invention in which the length of linear conductors that are electrically connected by a portion of the switch elements is the length of one half-wavelength of a desired transmission frequency, the positional relationship of linear conductors that are connected so as to be the length of xcex/2 of the transmission frequency to the linear radiation element can be shifted with respect to the longitudinal direction of the linear radiation element. By controlling this shift, the radiated beam can be directed either below or above the horizontal direction in the vertical plane.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.